Factors affecting calving to conception interval (days open) in dairy cows located at Dessie and Kombolcha towns, Ethiopia

This study was aimed at determining the median days of calving to conception interval (days open) and identifying the major risk factors determining the days open in dairy cows. Both retrospective and longitudinal studies were conducted on dairy cows located in the Dessie and Kombolcha towns of South Wollo Zone, northeast Ethiopia from August 2019 to August 2020. The Kaplan-Meier model of survival analysis was used to determine the median days open and compare the survival distribution of each level of explanatory variables likely to influence the days open of dairy cows. Accordingly, the overall conception rate of dairy cows was 44.7%. The median days open in the study was 154 days. There is a 16% probability of surviving (i.e, the probability that the conception event has not yet occurred) at the end of 210 days postpartum period. The Cox proportional hazard model was used to quantify the effect of each of the explanatory variables on the days open in the first 210 days postpartum. Factors that had a significant effect (p<0.05) on the calving to conception interval were the season of insemination, breeding system, calving to insemination interval, and herd milk yield level. However, the peripartum, postpartum disorders, and the town of the farm are not significant (p>0.05). Accordingly, cows inseminated in the autumn season (HR = 4.45), cows less than or equal to 85 days calving to insemination interval (HR = 2.41), artificially inseminated cows (HR = 1.45), and high herd milk yield cow had a higher probability of becoming pregnant. In conclusion, the management practices and breeding decisions seem to be important determinants to improve the conception rate or decrease the days open in dairy cows.

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Unfunded studies
Enter: The author(s) received no specific funding for this work. This study was carried out in cooperation with dairy farmers in the study areas and formalized by an official letter of cooperation obtained from the Research and Community Service Vice-President Office of Wollo University, Ethiopia. The purpose of the study was explained to all dairy farms included in the study and informed consent was sought from farm owners willing to be included in the study. The study was carried out under the existing dairy farm setting and the selection of dairy cows was for analysis cohorts. All interactions with the cows in this study were part of routine farm management where the researcher's involvement in this study was only observing and recording data in routine farm processes. Approval from an Institutional Animal Care and Use Committee (IACUC), or equivalent animal ethics committee, was not obtained as the researchers didn't manipulate cows or the experimental design led to differential treatment from routine practices on farms.

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Reproductive performance is the major concern of the modern dairy industry worldwide  were enrolled and followed for approximately 7 months/210 days. Cows that gave birth 120 less than 45 days at the initial visit and whose disease history and date of the last calving 121 known were recruited retrospectively (concurrent cohort) and allowed to join into the 122 prospective cohort. Other cows were recruited prospectively as they give birth within the 123 selected farms during the study period. Purchased or entrusted cows were not included in 124 this study. The recruited cows and those cows that gave birth after the initial visit were periods 5 to 50, 50 to 100, and 100 to 150 d after calving [22].

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The definitions of peri-and postpartum health disorders that were used in the present 188 study were similar to those described previously [22][23][24]. Calving difficulty was ranked 189 according to the degree of assistance required (1 = no assistance, 2 = minor assistance, 3 190 = some force required, 4 = significant force required, and 5 = cesarean section). Cows 191 with a calving score >2 were considered to have dystocia. The retained placenta was 192 defined as the retention of the fetal membranes for longer than 24 hours. Septicemic    days open was plotted for cows categorized by the season of insemination (Fig 2). The 254 plot revealed that the survival curve for cows inseminated in autumn is consistently 255 smaller than survival curves for cows inseminated in other seasons, respectively.  (Table 2). Accordingly, cows that have less than 80 days 269 calving to insemination interval had a higher probability (HR=2.50) to conceive than 270 cows that have greater than or equal to 80 days calving to insemination interval (Table 3). days open for Artificial insemination (AI) exposed cows ( used on days open. Compared to cows exposed to natural services, cows exposed to

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[7] revealed a higher chance of conception for heifers from herds with lower milk yield.

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Artificially inseminated cows had a higher probability of becoming pregnant than cows 347 exposed to natural service. These results of higher hazard rates for pregnancy in AI (DIM), and poor milk production. This suggests that the dairy producers believe that bull 354 breeding improves the likelihood that these non-pregnant and potentially lower producing 355 cows will become pregnant, and that the natural service (NS) breeding system lowers 356 their input costs to these less productive cows [38].

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The days open of dairy cows in Dessie and Kombolcha town of the study area was not respectively. In addition, cows with shorter calving to insemination interval and artificial 369 insemination exposed tend to have a greater probability to conceive than cows with 370 longer calving to insemination interval and natural service exposed cows. Herd milk yield